The Participation of GTP‐AMP‐P Transferase in Substrate Level Phosphate Transfer of Rat Liver Mitochondria1

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Abstract
From kinetic studies on the reaction sequence of substrate level phosphorylation in rat liver mitochondria, using anaerobic ketoglutarate dismutation in the presence of oligomycin and [32P]phosphate, phosphohistidine appears to be the first intermediate to be labelled, followed by GTP. [32P]ADP rather than [32P]ATP is shown to be the main product of the reaction. The phosphorylation of AMP requires ketoglutarate and is stimulated by 2,4‐dinitrophenol.GTP‐AMP‐P transferase is localized in the mitochondria. This conclusion is based on enzymatic assays of fractionally ectracted rat liver and of isolated mitochondria and microsomes.Mean values for the activities of GTP‐AMP‐P transferase, nucleoside diphosphate kinase and succinic thiokinase in rat liver mitochondria are given and are compared with the rate of ketoglutarate oxidation.A possible function of GTP‐AMP‐P transferase for the phosphorylation of endogenous AMP is discussed with regard to the compartmentation of nucleotides in the mitochondria.A new chromatographic assay for GTP‐AMP‐P transferase is reported, an assay which is not affected by nucleoside diphosphate kinase and adenylate kinase occurring in liver homogenates. An optical enzymatic assay for nucleoside diphosphate kinase is also described.